The sad sight of a orca mother carrying around her dead calf for days highlights the threat to the species as successful births plummet. Diminishing Chinook salmon stocks are largely to blame.A grieving orca whale continued for a sixth day on Sunday to carry her dead calf in Pacific Northwest waters, in a heart-wrenching spectacle that has focused attention on the plight of the endangered killer whale. Tahlequah, also known as J35, gave birth on Tuesday to a calf that only lived for a half-hour.The mother has since been pushing the dead calf with her rostrum, or forehead, to keep it on the water's surface.Read more: Microplastics endanger whales and sharksMembers of her pod have been staying close to Tahlequah during the entire drama near San Juan Island in the Salish Sea, off the coast of British Columbia and Washington State.“It is just unbelievable; she is still carrying her calf on her head, pushing it through the water. Her entire family is also staying close by with her,” Taylor Shedd of Soundwatch, which has been monitoring the pod, told the Seattle Times.“Her perfectly curved dorsal fin, her brilliant white saddle patch with the faintest of scratches … Her arching back glistening in the sun as she dives. It would be the most beautiful sight in the world if you didn't know what was happening,” he wrote in an email.Read more: Time to clean up the ocean garbage dumpThe calf was the first born in three years to the endangered Southern Resident Killer Whales, a clan of orcas that consist of three separate pods, each of which is further broken down into families centered around older females, often grandmothers and great-grandmothers.There are only 75 whales left in the three pods, the lowest level in nearly three decades and down from 98 in 1995.Researchers say that the orcas are threatened by marine noise from boats, water pollution and the decline of Chinook salmon populationsUnlike other orcas, Southern Resident Killer Whales feed almost exclusively on the fatty fish whose populations have fallen due to overfishing and the damming and degradation of rivers up which they spawn.As a result of nutrition deficiency female orcas have not had successful pregnancies, with all failing in the past three years.A University of Washington study found that two-thirds of orcas pregnancies failed between 2007 and 2014.“On average we expect a few calves born each year. The fact that we haven't seen any in several years and then to have reproductive failure is further evidence that we have a severe problem with the reproductive viability in the population,” Brad Hanson, a wildlife biologist with the Northwest Fisheries Science Center in Seattle, told the Associated Press.A short baby boom starting in December 2014 that came with bountiful salmon runs led to 11 orcas being born, but about half of them have since died.Read more: Cod under threat in the BalticThe first orca born in that boom, a 4-year-old female orca known as J-50, is now sick and emaciated, researchers said.Another factor that may be hitting orca populations is the health of older, post-menopausal females.Orca females usually have their last offspring in their late 30s or early 40s, but can live up to 100 years.The Center for Whale Research, which studies and monitors Southern Resident Killer Whales, has found that post-reproductive females help in the survival rates of their adult sons and daughters.They do this in part by using their knowledge of salmon foraging grounds, especially in times of low salmon. Once a post-reproductive female dies, the risk of an adult son dying is eight times higher, while that of an adult female increases 2.7 fold.